Processes for the production of chlorine and sodium hydroxide have changed dramatically in the past 20 to 25 years. At the start of that time period a large percentage of the chlorine and caustic soda (sodium hydroxide solution) produced in the United States was produced by the electrolytic decomposition of sodium chloride brine in mercury cells and diaphragm cells. The development of the membrane cell led to the virtual elimination of mercury cells and many diaphragm cell installations have been partially or completely converted to membrane cell installations. Also new installations now employ membrane cells, rather than the other types used in the past. The use of membrane cells for electrolytic decomposition have many advantages over the older cells. Among these advantages are elimination of mercury and asbestos pollution problems, reduced power consumption, improved cell efficiency, and better quality of products.
The use of membrane cells also brings a new set of problems. Impurities in the aqueous sodium chloride brine fed to the cells cannot be tolerated in membrane cells to the degree they could be tolerated in diaphragm or mercury cells, because membranes are more easily plugged than are asbestos deposited diaphragms. For example, a few parts of calcium ions per million parts of brine could be tolerated in the diaphragm type cells. This amount of calcium in the feed brine to membrane cells will damage the membranes severely, causing a major drop in cell efficiency with a corresponding increase in power consumption. Also, metallic impurities, such as iron and copper are detrimental in the brine being electrolyzed in membrane cells and must be removed or minimized. Further, while diaphragm cells can perform well with significant amounts of sodium sulfate in the feed brine to the cells, most membrane cells require a sodium sulfate level of less than 8 grams per liter in the feed brine to the cells. Depending upon the operating conditions, levels higher than about 8 grams per liter can contribute to lower efficiencies, including higher power costs, damage to the anode coatings and in some cases, damage to the membranes.
In the operation of diaphragm cells, the major operating cost factors are power, steam, and brine. Membrane cells have dramatically reduced power and steam costs but brine (salt) costs are much more significant because the membranes require high purity brine to operate efficiently. This necessitates the removal of the buildup of certain impurities in the system, such as those referred to above, which are harmful to the performance of the membrane cells.
The source of sodium chloride used in membrane cells may be rock salt, solar salt, byproduct salt from an existing diaphragm cell plant, also known as caustic process or C.P. Salt, purified evaporative salt or brine delivered in a pipeline which has been solution mined from underground salt deposits. Among the factors that influence the selection of the brine/salt source are quality, freight cost, capital cost, disposal costs of impurities that build up in the in the evaporative system (such as bitterns which are waste brines containing 40 grams per litter of sodium sulfate or more, and must be removed for efficient operation), and reliability of the source of supply. Thus, selection of the source of the sodium chloride raw material is an important, and sometimes a critical factor when considering a membrane cell installation. Rock salt, solar salt and diaphragm cell recovered caustic process salt (C.P. salt) contain an undesirable amount of impurities, and they are chosen only when the cost of purer salt (or brine) is too high. Certain plant sites, particularly in New York, Texas, Louisiana, Mississippi and Alabama can be located close enough to underground deposits of salt solution mined at brine wells to make the use of pipeline brine highly desirable. However, this source of brine/salt also has associated problems. Nearly saturated sodium chloride brine from wells contains roughly three pounds of water per pound of sodium chloride. This saturated brine is purified by conventional brine treatment and evaporation methods, followed by further purification in ion exchange units before it is used in membrane cells. It is then acidified and fed to the cells where almost half of the sodium chloride in the feed brine is decomposed to make the products of the electrolysis, i.e., liquid sodium hydroxide, and, gaseous chlorine and hydrogen. The brine exiting the cells consists of approximately 80 weight percent of the water contained in the feed brine and the remaining salt which was not decomposed by the electrolytic process taking place in the cells. This results in an outflow of depleted brine from the cells, usually in the range of 16-18 weight percent sodium chloride (NaCl). Environmental and economic considerations make it impractical to dispose of the depleted brine. Accordingly, various processes and systems have been proposed for treating the depleted brine so it may be recycled to the electrolytic cells as further described in the Prior Art which follows.